816 + Interview Questions in MAGNETISM IN GENERAL AWARENESS Page 13 InterviewSolution

601.	The angle of dip at the magnetic equator isA. `0^(o)`B. `45^(o)`C. `30^(o)`D. `90^(o)`
Answer» Correct Answer - A

Discussion

602.	The angle of dip at the equator is (A) 90° (B) 45° (C) 30° (D) 0°
Answer» Correct option is: (D) 0°

Discussion

603.	The geometric length of a bar magnet is `24cm`. The length of the magnet isA. `24cm`B. `28.8cm`C. `20cm`D. `30cm`
Answer» Correct Answer - C `2l=(5)/(6)["geometric length"]`

Discussion

604.	The geometric length of a bar magnet having half magnetic length 5 cm is ……… cm.(A) 12 (B) 10 (C) 6 (D) 4.2
Answer» Correct option is: (A) 12

Discussion

605.	A magnetic dipole has magnetic length 10 cm and pole strength 100 Am. Its magnetic dipole moment is ……… Am2.(A) 1000 (B) 500 (C) 10 (D) 5
Answer» Correct option is: (C) 10

Discussion

606.	A magnet with moment M is given. If it is bent into a semicircular form, its new magnetic moment will be:A. MB. 2MC. `2M//pi`D. `M//2pi`
Answer» Correct Answer - C `Radius=r,therefore l=pi r therefore r=(1)/(pi)` Distance between two poles`=2 r=(2l)/(pi)` pole strength `m=(M)/(l)` Thus magnetic moment`=(M)/(l)(2l)/(pi)=(2M)/(pi)`

Discussion

607.	A long thin magnet of moment `M` is bent into a semi circle. The decrease in the magnetic moment isA. `4 pi A-m^(2) `B. `8 pi A-m^(2)`C. `4A-m^(2) `D. None of these
Answer» Correct Answer - D The magnetic moment will be zero because it becomes an open circuit .

Discussion

608.	A long thin magnet of moment `M` is bent into a semi circle. The decrease in the magnetic moment isA. `2M//pi`B. `piM//2`C. `M(pi-2)//pi`D. `M(2-pi)//2`
Answer» Correct Answer - C For semi circle, `M^(1)=(2M)/(pI)` decrease in `M,DeltaM=M-M^(1)`

Discussion

609.

Write down the difference between Coulomb’s Law and Biot-Savart Law

Answer»

S. No.	Electric field	Magnetic field
1.	Produced by a scalar source i.e., and electric charge q	Produced by a vector source i.e., current element I \(\vec {dl}\)
2.	It si directed along the position vector joining the source and the point at which the field is calculated	it is directed perpendicular to the position vector \(\vec r\)and the current element I \(\vec{dl}\)
3.	Does not depend on angle	Depends on the angle between the position vector \(\hat r\) and the current I \(\vec{dl}\)

Discussion

610.	What is magnetic permeability?
Answer» Magnetic permeability: The magnetic permeability can be defined as the measure of ability of the material to allow the passage of magnetic field lines through it or measure of the capacity of the substance to take magnetisation or the degree of penetration of magnetic field through the substance

610.

What is magnetic permeability?

Answer»

Magnetic permeability:

The magnetic permeability can be defined as the measure of ability of the material to allow the passage of magnetic field lines through it or measure of the capacity of the substance to take magnetisation or the degree of penetration of magnetic field through the substance

Discussion

611.	Figure shows a loop model (loop `L`) for a diamgnetic material. A. the net dipole moment of the loop directed towards the magnetB. The net dipole moment of the loop directed away from the loopC. The loop gets attracted towards the magnetD. The loop gets repelled by the magnet
Answer» Correct Answer - B::D The near face of the loop behaves like south pole and far face as north pole. So loop will be repelled by the magnet.

Discussion

612.	A rod of a paramagnetic substance is placed in a non-uniform magnetic field. Which of the following figure shows its alignment in the field?A. B. C. D.
Answer» Correct Answer - A

Discussion

613.	Assertion : The properties of paramagnetic and ferromagnetic substance are not effected by heating. Reason : As temperature rises, the alignment of molecular magnets gradually decreases.A. If both assertion and reason are true and the reason is the correct explanation of the assertion.B. If both assertion and reason are true but reason is not the correct explanation of the assertionC. if assertion is true but reason is false.D. If assertion is false but reason is true
Answer» Correct Answer - D

Discussion

614.	Ferromagnetic ore properties are due toA. filled inner sub-shellsB. vacant inner sub-shellsC. partially filled inner sub-shellsD. all the sub-shells equally filled
Answer» Correct Answer - C

Discussion

615.	Ferromagnetic ore properties are due toA. vacant inner subshellsB. partically filled inner subshellsC. filled inner subshellsD. completely filled outer shell
Answer» Correct Answer - B

Discussion

616.	If a diamagnetic solution is poured into a U-tube and one aem of this U-tube placed between the poles of a strong magnet with the meniscus in a line with the field, then the level of the solution willA. fallB. riseC. oscillateD. remain unchanged
Answer» Correct Answer - A

Discussion

617.	Assertion (A): If one arm of a `U-`tube containing a dia magnetic solution is placed in between the poles of a strong magnet with the level in line withh the field, the level of the solution falls, Reason (R ): Diamagnetic substances areA. Both `A` and `R` are true and `R` is the correct explanation of `A`.B. Both `A` and `R` are true and `R` is not correct explanation of `A`.C. `A` is true, But `R` is falseD. `A` is false, But `R` is true
Answer» Correct Answer - A

Discussion

618.	The angle of dip at the poles and the equator respectively areA. `30^(@),60^(@)`B. `90^(@),0^(@)`C. `30^(@),90^(@)`D. `0^(@),0^(@)`
Answer» Correct Answer - B

Discussion

619.	Two bar magnets are bound together side by side and suspended. They swing in `12s` when their like poles are together and in `16s` when their unlike poles are together, the magnetic moments of these magnets are in the ratioA. `27:5`B. `25:7`C. `7:25`D. `24:7`
Answer» Correct Answer - B `T=2pisqrt((I)/(MB_(H)))` and `(T_(1))/(T_(2))=sqrt((M_(1)-M_(2))/(M_(1)+M_(2))`

Discussion

620.	Assertion (A): The net magnetic flux coming out of a closed surface is always zero. Reason (R ): Unlike poles of equal strength exist togetherA. Both `A` and `R` are true and `R` is the correct explanation of `A`.B. Both `A` and `R` are true and `R` is not correct explanation of `A`.C. `A` is true, But `R` is falseD. `A` is false, But `R` is true
Answer» Correct Answer - B

Discussion

621.	Find the wrong statement among the following. Two unlike isolated magnetic poles are at some distance apart in air.A. the resulting induction at a point between the poles is `B_(1)+B_(2)` on the line joining themB. The resultant induction is `B_(1)~B_(2)` at any point out side the poles on the line joining themC. No neutral point is formed on the line joining them if the pole strengths are equal.D. A neutral point is formed in between the poles and nearer to weak pole on the line joining them.
Answer» Correct Answer - D

Discussion

622.	A short bar magnet placed with its axis at `30^(@)` with a uniform external magnetic field of 0.35 T experiences a torque of magnitude equal to `4.5xx10^(-2)Nm`. The magnitude of magnetic moment of the given magnet isA. 1.36 J/TB. 0.06 J/TC. 0.36 J/TD. 0.60 J/T
Answer» Correct Answer - C `tau=MB sintheta` `M=(tau)/(Bsintheta)=(4.5xx10^(-2))/(0.25xxsin30)=0.36 Am^(2)`

Discussion

623.	Magnetic intensity for an axial point due to a short bar magnet of magnetic moment `M` is given byA. `(mu_(0))/(4pt)xx(M)/(d^(3))`B. `(mu_(0))/(4pi)xx(M)/(d^(2))`C. `(mu_(0))/(2pi)xx(M)/(d^(2))`D. `(mu_(0))/(2pi)xx(M)/(d^(2))`
Answer» Correct Answer - C

Discussion

624.	Due to a small magnet intensity at a distance `x` in the end on position is `9` Gauss. What will be the intensity at a distance `(x)/(2)` on broad side on position?A. 9 gaussB. 4 gaussC. 36 gaussD. 4.5 gauss
Answer» Correct Answer - C In CGS`,B_("axial")=9=(2M)/(x^(3))` ...........(i) ` B_("equatorial")=(M)/(((x)/(2))^(2))=(8M)/(x^(3))` ........(ii) From Eq. (i) and (ii) , `B_("equatorial")=36`gauss

Discussion

625.	The magnetic field at a distance d from a short bar magnet in longitudinal and transverse positions are in the ratio.A. `1:1`B. `2:3`C. `2:1`D. `3:2`
Answer» Correct Answer - C For longitudinal positions, magnetic field will be `B_(1) prop (2M)/(d^(3))` . For transverse positions magnetic field will be `B_(2) prop (M)/(d^(3))` . `:. (B_(1))/(B_(2))=2:1 `

Discussion

626.	At a point on the right bisector of a magnetic dipole the magnetic potentialA. potential varies as ` 1//r^2 `B. potential is zero at all points on the right bisectorC. field varies as ` r^(2)`D. field is perpendicular to the axis of dipole
Answer» Correct Answer - B Magnetic potential is zero at all the points on the right bisector.

Discussion

627.	At a point on the right bisector of a magnetic dipole the magnetic potentialA. potential varies as `(1)/(r^(2))`B. Potential is zero at all points on the right bisectorC. field varies as `r^(2)`D. Field is perpendicular to the axis of dipole
Answer» Correct Answer - A `V=(mu_(0))/(4pi)(MaStheta)/(r^(2))` At right side of bisector,`theta=0` `therefore V oo(1)/(r^(2))`

Discussion

628.	A circular coil of radius r is fromed by wire of length L.If current I is flowing through it then the magnetic moment Is proportional toA. `L^(2)`B. LC. `L^(3)`D. `L^(1//2)`
Answer» Correct Answer - A `L=2pir` `therefore r=(L)/(2pi)` `M=IA=Ipir^(2)` `M=IA=Ipir^(2)` `M=Ipi(L^(2))/(4pi)=(IL^(2))/(4pi)` `Moo L^(2)`

Discussion

629.	Two shrt magnets with their axes horizontal and perpendicular to the magnetic meridian are placed with their centres `40 cm` east and `50 cm` west of magnetic needle. If the needle remains undeflected, the rato of their magnetic moment `M_(1): M_(2)` isA. `4:5`B. `16:25`C. `64:125`D. `2:sqrt(5)`
Answer» Correct Answer - C

Discussion

630.	A current loop placed in a magnetic field behaves like aA. Magnetic dipoleB. Magnetic substanceC. Magnetic poleD. All are true
Answer» Correct Answer - A

Discussion

631.	Assertion: When radius of circular loop carrying current is doubled, its magnetic moment becomes four times. Rrason: Magnetic moment depends on area of the loop.A. If both assertion and reason are true and the reason is the correct explanation of the assertion.B. If both assertion and reason are true but reason is not the correct explanation of the assertionC. f assertion is true but reason is false.D. If the assertion and r eason both are false
Answer» Correct Answer - B

Discussion

632.	The gyromagnetic ratio of electron isA. `8.8xx10^(10) C//kg`B. `4.8xx10^(10)C//Kg`C. `6.8xx10^(10)C//kg`D. `5.8xx10^(10 )C//Kg`
Answer» Correct Answer - A

Discussion

633.	Assertion: When radius of circular loop carrying current is doubled, its magnetic moment becomes four times. Rrason: Magnetic moment depends on area of the loop.A. If both assertion and reason are true and the reason is the correct explanation of the assertion.B. If both assertion and reason are true but reason is not the correct explanation of the assertionC. f assertion is true but reason is false.D. If the assertion and r eason both are false
Answer» Correct Answer - B

Discussion

634.	Magnetic moment of coil independent ofA. Area of coilB. Number of turnC. Stregth of magnetic fieldD. Area of coil
Answer» Correct Answer - C

Discussion

635.	A compass is used for …………(a) plotting magnetic lines (b) detection of magnetic field (c) navigation (d) All of these
Answer» (d) All of these

Discussion

636.	The product of ampere turn and area of coil isA. Magnetic dipole momentB. TorqueC. Magnetic potentialD. Magnetic induction
Answer» Correct Answer - A

Discussion

637.	The pole strength of each pole of a short bar magnet having magnetic length 2 cm and the magnetic field produced by this magnet at a point 20 cm from the centre of the magnet on the line making an angle of `60^(@)` with the axis is `3.75 xx10^(-5) Wb//m^(2)`,isA. 213.4 AmB. 103.4 AmC. 111.4 AmD. 113.4Am
Answer» Correct Answer - D `B=(mu_(0))/(4pi)(M)/(r^(3))sqrt(3cos^(2)60+1)` `3.75xx10^(-5)=(10^(-7)xx2xx10^(-2)xxm)/(8xx10^(-3))sqrt(3cos^(2)60+1)` `m=113.4Am`

Discussion

638.	Current carrying circular loop acts asA. Magnetic dipoleB. Magnetic fluxC. Magnetic fieldD. Condenser
Answer» Correct Answer - A

Discussion

639.	MRI stands for……… (a) Magnetic Running Image (b) detection of magnetic field (c) navigation (d) Magnetic Radar Imaging
Answer» (a) Magnetic Resonance Imaging

Discussion

640.	The product of nuber of turns,area of coil and current isA. Electric dipoleB. Magnetic dipole momentC. Magnetic dipoleD. Magnetic potential
Answer» Correct Answer - B

Discussion

641.	The product of number of turns and current of loop isA. Ampere turnB. Flux densityC. ChargeD. None of these
Answer» Correct Answer - A

Discussion

642.	A bar magnet produces a field which is similaar to a field produced by current flowing throughA. A circular coilB. Straight conductorC. A rectangular coilD. A solenoid
Answer» Correct Answer - D

Discussion

643.	The shape of the Earth’s magnetic field resembles that of an imaginary …………(a) U-shaped magnet (b) straight conductor carrying current (c) solenoid coil (d) bar magnet
Answer» (d) bar magnet

Discussion

644.	The magnetic lines of forceA. Do not intersectB. Intersects at infinityC. Intersects within the magnetD. Intersects at neutral point
Answer» Correct Answer - A

Discussion

645.	Write a note on Earth’s magnetism.
Answer» 1. Earth has been assumed or imagined by the scientists as a huge magnetic dipole. 2. The south pole of the imaginary magnet inside the Earth is located near the geographic north pole and the north pole of the Earth’s magnet is located near the geographic south pole. 3. The line joining these magnetic poles is called the magnetic axis. 4. The magnetic axis intersects the geographic north pole at a point called the north geomagnetic pole or northern magnetic pole. 5. It intersects the geographic south pole at a point called the south geomagnetic pole or southern magnetic pole. 6. The magnetic axis and the geographical axis (axis of rotation) do not coincide with each other. 7. The cause of the Earth’s magnetism, are as follows. Masses of magnetic substances in the Earth Radiations from the Sun Action of the Moon

645.

Write a note on Earth’s magnetism.

Answer»

1. Earth has been assumed or imagined by the scientists as a huge magnetic dipole.

2. The south pole of the imaginary magnet inside the Earth is located near the geographic north pole and the north pole of the Earth’s magnet is located near the geographic south pole.

3. The line joining these magnetic poles is called the magnetic axis.

4. The magnetic axis intersects the geographic north pole at a point called the north geomagnetic pole or northern magnetic pole.

5. It intersects the geographic south pole at a point called the south geomagnetic pole or southern magnetic pole.

6. The magnetic axis and the geographical axis (axis of rotation) do not coincide with each other.

7. The cause of the Earth’s magnetism, are as follows.

Masses of magnetic substances in the Earth
Radiations from the Sun
Action of the Moon

Discussion

646.	The electric intensity at any point due to point charge is `E=(1)/(4piin_(0))xx(q)/(r^(2)`.In analogy with this,What will be magnetic iduction at a point due to a magnetic charge?A. `(pi_(0))/(4pi) (m)/(r^(2))`B. `(pi_(0))/(4pi) (m^(2))/(r^(2))`C. `(pi_(0))/(2pi) (m)/(r^(2))`D. `(pi_(0))/(4pi) r^(2)/(m)`
Answer» Correct Answer - A

Discussion

647.	The elctric potential at any point due to a point charge is `v=(1)/(4piin_(0)) (q)/(r)`.In analogy with this ,What will be magnetic potential at any point due to a magnetic charge?A. `(pi_0)/(4pi) (m^(2))/(r)`B. `(pi_(0))/(4pi) (m)/(r)`C. `(pi_(0))/(4pi) (m)/(r^(2))`D. `(pi_(0))/(4pi) (r^(2))/(m)`
Answer» Correct Answer - B

Discussion

648.	The systerm of two fictitious poles separated by certain distance is called asA. Electric dipoleB. Magnetic dipoleC. Magnetic dipole momentD. None of these
Answer» Correct Answer - B

Discussion

649.	Write a short note on the theory of magnetism.
Answer» All substances are made of minute particles. In a magnetic substance whether magnetized or non-magnetized), each individual particle behaves like an independent magnet. In the unmagnetized state, these particles within the substance are present in a random fashion such that they cancel out the magnetic effects of each other. In a magnetized state, these particles align in a particle direction and the substance behaves like a magnet.

Discussion

650.

What are some properties of magnetic lines of force?

Answer»

1. Magnetic lines of force originate from the north pole and end at the south pole.

2. The magnetic lines of force of a magnet or a solenoid form closed loops. This is in contrast to the case of an electric dipole, where the electric lines of force originate from the positive charge and end on the negative charge.

3. The direction of the net magnetic field \(\overset\rightarrow{B}\) at a point is given by the tangent to the magnetic line of force at that point.

4. The number of lines of force crossing per unit area decides the magnitude of magnetic field \(\overset\rightarrow{B}.\)

5. The magnetic lines of force do not intersect. This is because had they intersected, the direction of magnetic field would not be unique at that point.

Discussion

Explore topic-wise InterviewSolutions in .

The angle of dip at the magnetic equator isA. `0^(o)`B. `45^(o)`C. `30^(o)`D. `90^(o)`

The angle of dip at the equator is (A) 90° (B) 45° (C) 30° (D) 0°

The geometric length of a bar magnet is `24cm`. The length of the magnet isA. `24cm`B. `28.8cm`C. `20cm`D. `30cm`

The geometric length of a bar magnet having half magnetic length 5 cm is ……… cm.(A) 12 (B) 10 (C) 6 (D) 4.2

A magnetic dipole has magnetic length 10 cm and pole strength 100 Am. Its magnetic dipole moment is ……… Am2.(A) 1000 (B) 500 (C) 10 (D) 5

A magnet with moment M is given. If it is bent into a semicircular form, its new magnetic moment will be:A. MB. 2MC. `2M//pi`D. `M//2pi`

A long thin magnet of moment `M` is bent into a semi circle. The decrease in the magnetic moment isA. `4 pi A-m^(2) `B. `8 pi A-m^(2)`C. `4A-m^(2) `D. None of these

A long thin magnet of moment `M` is bent into a semi circle. The decrease in the magnetic moment isA. `2M//pi`B. `piM//2`C. `M(pi-2)//pi`D. `M(2-pi)//2`

Write down the difference between Coulomb’s Law and Biot-Savart Law

What is magnetic permeability?

Figure shows a loop model (loop `L`) for a diamgnetic material. A. the net dipole moment of the loop directed towards the magnetB. The net dipole moment of the loop directed away from the loopC. The loop gets attracted towards the magnetD. The loop gets repelled by the magnet

A rod of a paramagnetic substance is placed in a non-uniform magnetic field. Which of the following figure shows its alignment in the field?A. B. C. D.

Ferromagnetic ore properties are due toA. filled inner sub-shellsB. vacant inner sub-shellsC. partially filled inner sub-shellsD. all the sub-shells equally filled

Ferromagnetic ore properties are due toA. vacant inner subshellsB. partically filled inner subshellsC. filled inner subshellsD. completely filled outer shell

If a diamagnetic solution is poured into a U-tube and one aem of this U-tube placed between the poles of a strong magnet with the meniscus in a line with the field, then the level of the solution willA. fallB. riseC. oscillateD. remain unchanged

The angle of dip at the poles and the equator respectively areA. `30^(@),60^(@)`B. `90^(@),0^(@)`C. `30^(@),90^(@)`D. `0^(@),0^(@)`

Two bar magnets are bound together side by side and suspended. They swing in `12s` when their like poles are together and in `16s` when their unlike poles are together, the magnetic moments of these magnets are in the ratioA. `27:5`B. `25:7`C. `7:25`D. `24:7`

A short bar magnet placed with its axis at `30^(@)` with a uniform external magnetic field of 0.35 T experiences a torque of magnitude equal to `4.5xx10^(-2)Nm`. The magnitude of magnetic moment of the given magnet isA. 1.36 J/TB. 0.06 J/TC. 0.36 J/TD. 0.60 J/T

Magnetic intensity for an axial point due to a short bar magnet of magnetic moment `M` is given byA. `(mu_(0))/(4pt)xx(M)/(d^(3))`B. `(mu_(0))/(4pi)xx(M)/(d^(2))`C. `(mu_(0))/(2pi)xx(M)/(d^(2))`D. `(mu_(0))/(2pi)xx(M)/(d^(2))`

Due to a small magnet intensity at a distance `x` in the end on position is `9` Gauss. What will be the intensity at a distance `(x)/(2)` on broad side on position?A. 9 gaussB. 4 gaussC. 36 gaussD. 4.5 gauss

The magnetic field at a distance d from a short bar magnet in longitudinal and transverse positions are in the ratio.A. `1:1`B. `2:3`C. `2:1`D. `3:2`

At a point on the right bisector of a magnetic dipole the magnetic potentialA. potential varies as ` 1//r^2 `B. potential is zero at all points on the right bisectorC. field varies as ` r^(2)`D. field is perpendicular to the axis of dipole

At a point on the right bisector of a magnetic dipole the magnetic potentialA. potential varies as `(1)/(r^(2))`B. Potential is zero at all points on the right bisectorC. field varies as `r^(2)`D. Field is perpendicular to the axis of dipole

A circular coil of radius r is fromed by wire of length L.If current I is flowing through it then the magnetic moment Is proportional toA. `L^(2)`B. LC. `L^(3)`D. `L^(1//2)`

Two shrt magnets with their axes horizontal and perpendicular to the magnetic meridian are placed with their centres `40 cm` east and `50 cm` west of magnetic needle. If the needle remains undeflected, the rato of their magnetic moment `M_(1): M_(2)` isA. `4:5`B. `16:25`C. `64:125`D. `2:sqrt(5)`

A current loop placed in a magnetic field behaves like aA. Magnetic dipoleB. Magnetic substanceC. Magnetic poleD. All are true

The gyromagnetic ratio of electron isA. `8.8xx10^(10) C//kg`B. `4.8xx10^(10)C//Kg`C. `6.8xx10^(10)C//kg`D. `5.8xx10^(10 )C//Kg`

Magnetic moment of coil independent ofA. Area of coilB. Number of turnC. Stregth of magnetic fieldD. Area of coil

A compass is used for …………(a) plotting magnetic lines (b) detection of magnetic field (c) navigation (d) All of these

The product of ampere turn and area of coil isA. Magnetic dipole momentB. TorqueC. Magnetic potentialD. Magnetic induction

Current carrying circular loop acts asA. Magnetic dipoleB. Magnetic fluxC. Magnetic fieldD. Condenser

MRI stands for……… (a) Magnetic Running Image (b) detection of magnetic field (c) navigation (d) Magnetic Radar Imaging

The product of nuber of turns,area of coil and current isA. Electric dipoleB. Magnetic dipole momentC. Magnetic dipoleD. Magnetic potential

The product of number of turns and current of loop isA. Ampere turnB. Flux densityC. ChargeD. None of these

A bar magnet produces a field which is similaar to a field produced by current flowing throughA. A circular coilB. Straight conductorC. A rectangular coilD. A solenoid

The shape of the Earth’s magnetic field resembles that of an imaginary …………(a) U-shaped magnet (b) straight conductor carrying current (c) solenoid coil (d) bar magnet

The magnetic lines of forceA. Do not intersectB. Intersects at infinityC. Intersects within the magnetD. Intersects at neutral point

Write a note on Earth’s magnetism.

The elctric potential at any point due to a point charge is `v=(1)/(4piin_(0)) (q)/(r)`.In analogy with this ,What will be magnetic potential at any point due to a magnetic charge?A. `(pi_0)/(4pi) (m^(2))/(r)`B. `(pi_(0))/(4pi) (m)/(r)`C. `(pi_(0))/(4pi) (m)/(r^(2))`D. `(pi_(0))/(4pi) (r^(2))/(m)`

The systerm of two fictitious poles separated by certain distance is called asA. Electric dipoleB. Magnetic dipoleC. Magnetic dipole momentD. None of these

Write a short note on the theory of magnetism.

What are some properties of magnetic lines of force?